Various filters are used for a filter system which is to purify water. Representative examples of the filters include a reverse osmosis membrane filter and a hollow fiber membrane filter.
The reverse osmosis membrane filter refers to a filter that reversely uses osmosis phenomenon. In a heavily doped solution and a lightly doped solution, which are separated from each other by a semi-permeable membrane, water is moved from the lightly doped solution to the heavily doped solution by naturally passing through the semi-permeable membrane. This phenomenon will be referred to as osmosis phenomenon, and at this time, a water level difference between the heavily doped solution and the lightly doped solution will be referred to as an osmotic pressure. If a pressure more than the osmotic pressure is given to the heavily doped solution, water is moved from the heavily doped solution to the lightly doped solution by passing through the semi-permeable membrane on the contrary to the natural phenomenon. This phenomenon will be referred to as reverse osmosis phenomenon, and at this time, a water level difference between the lightly doped solution and the heavily doped solution will be referred to as a reverse osmotic pressure. The reverse osmosis membrane filter is comprised to purify water by allowing water molecules only to pass through the semi-permeable membrane.
The hollow filter membrane filter is based on a thread-like filter of which center portion is empty, such as a bamboo. Pores are formed in the hollow filter membrane filter to filter target materials to be removed, which are mixed with water, and pass through water molecules. If water passes through the hollow filter membrane filter by using a water pressure, target materials to be removed, which are greater than the pores, fail to pass through the pores, and the water molecules smaller than the pores may pass through the hollow filter membrane filter. The hollow filter membrane filter is comprised to purify raw water by using the principle described as above. However, it is known that the hollow filter membrane filter fails to filter finer materials as compared with the reverse osmosis membrane filter.
Viruses of target materials to be removed from the raw water are formed at a fine size invisible to the naked eye. Particularly, if viruses, such as Noro viruses, which adversely affect a human's body, are contained in drinking water, such viruses cause a stomachache, whereby it is essentially required to remove the viruses from the filter system. However, since the viruses are formed at a fine size, it is general that the reverse osmosis membrane filter is more effective to remove fine materials than the hollow fiber membrane filter. Therefore, the reverse osmosis membrane filter has been generally used to remove viruses from raw water.
However, the applicant has devised a hollow fiber membrane, which may remove viruses, through studies and development of the hollow fiber membrane. Since the hollow fiber membrane, which may remove viruses, has pores of which sizes are smaller than those of the viruses, a problem has been raised in that a discharge capacity is rapidly reduced due to nanoparticles existing in water with the passage of time.
Therefore, a filter system, which may solve the problem that a discharge capacity is rapidly reduced due to nanoparticles when a hollow fiber membrane for removing viruses is used, may be considered.